A common method of mining alluvial deposits, such as those in and around river beds, is to utilize a floating dredge or pontoon having an excavating device positioned at one end thereof. The excavating device removes material from the bed of the river or other body of water in which it is working, and delivers the material to the pontoon. There it is processed to separate the desired minerals or other pay material from waste material. The waste material is then transported off the pontoon, normally to reclaim the area which has just been dredged.
This method of mining alluvial deposits has been in use for well over one hundred years, as evidenced by U.S. Pat. No. 285,565 to Brotherhood. The Brotherhood patent discloses a floating pontoon having an endless bucket line conveyor for excavating material from underwater and directing it to the pontoon. An endless bucket line is very often used in mining of this type since the dredging is continuous and a high capacity of material is capable of being removed.
Other apparatus and methods of dredging ore utilize a hydraulic suction device or a bucket line type conveyor in combination with a hydraulic suction device. U.S. Pat. No. 1,148,816 to Alleman illustrates the use of a suction device for underwater dredging. The Alleman patent also shows the use of an agitating belt to loosen the dredged material so that it can be received into the suction pipe.
U.S. Pat. No. 748.804 to Smith et al. illustrates the use of a bucket line conveyor and hydraulic suction device in combination. The suction device transports fine material, while the bucket line excavates and transports larger material. The in-feed ends of both dredging systems are adjacent to one another. Both are carried by the same support frame and move in unison with respect to the supporting pontoon.
Some alluvial type deposits have as much as 90 to 100 feet of material above bedrock. Very often, particularly in the case of gold or other heavy metals, most all of the desired minerals are found in the lower 20% or so of the material overlying the bedrock. Accordingly, in a deposit with one hundred feet of total material overlying bedrock, most all of the gold bearing material would be in the lower twenty feet. This leaves a total of eighty feet of overburden material overlying what is commonly referred to as the "pay zone" containing the gold. To get to that pay material, the overburden must first be removed. Accordingly, there is approximately four times the volume of waste overburden material which needs to be removed with respect to the volume of pay material.
Prior art dredging devices, such as those just described, can be used to excavate this large volume of material. However, where the proportion of overburden material to that of the pay material is great, a large amount of waste material is processed by the dredge before ever reaching the pay material. That is, all material is treated identically to extract the desired minerals from the waste material. This means that in the example discussed above, for every cubic yard of pay material processed there are four cubic yards of overburden material also processed. Because of this, a large percentage of the material processing time is wasted in merely cycling overburden material through the processors so that it can be eventually removed from the barge.
It should thus be readily apparent that such prior art devices and methods of dredging are extremely inefficient in excavating deposits having a large amount of overburden material in proportion to underlying pay material. Accordingly, a need remains for improved methods and apparatus which are capable of mining such deposits in an efficient manner.